Counterfeiting of security documents, especially of banknotes, is and remains a major concern for the industry and the economy around the world. Most counterfeited banknotes are produced using common imaging and printing equipment that is readily available to any user on the consumer market. The advent of scanners and colour copiers, as well as high-resolution colour printers making use of widespread printing processes, such as ink-jet printing, thermal printing and laser printing, makes it easier and easier to produce substantial volumes of counterfeited security papers. Most banknote counterfeits are produced by means of the above-mentioned imaging and printing equipment and can be designated as “colour copies”.
Offset-printed forgeries, or “offset counterfeits” printed using commercial offset printing presses do also exist. These counterfeits are often printed in screen offset (i.e. with multicolour screen or raster combinations that are characteristic of commercial offset printing) and/or line offset (i.e. without any screen or raster combinations).
Most genuine banknotes combine high quality printed features created by intaglio printing, line offset printing with high precision recto-verso register, and letterpress printing. Intaglio and line offset in particular allow the creation of high resolution patterns with great print sharpness. Letterpress printing is typically used for printing variable information, such as serial numbers. Further printing or processing techniques are also exploited to print or apply other features on banknotes, such as silk-screen printing, foil stamping, laser marking or perforating, etc.
Skilled persons having some knowledge of the processes involved in the context of the production of banknotes and like security documents do not as such have much difficulty in differentiating most forged documents from a genuine document. A close look at a forged document using simple means such as a magnifying glass typically makes it possible to immediately identify the characteristic features intrinsic to genuine security documents, such as the intaglio-printed security patterns that are present on most banknotes as already mentioned. This however requires some expertise and knowledge about security printing which is not necessarily present amongst the public at large. In practice, most individuals are relatively easily deceived by forgeries as long as the general look of the counterfeit or copy is substantially similar to that of the genuine document. This represents not only a problem in the context of banknote counterfeiting, but also as regards forgery of other types of valuable documents, such as checks, duty stamps, identification and travel documents, etc.
Machine-based authentication of security documents, i.e. automatic recognition in document processing systems such as vending machines, automatic teller machines (ATM), note acceptors and similar financial transaction machines, is also affected by counterfeiting. Indeed, it is not unusual to discover rather more advanced forgeries of security documents which also replicate the machine-readable security features present on genuine documents, such as infrared, luminescent and/or magnetic markings. As a matter of fact, most machine-based authentication systems essentially focus on such machine-readable features and do not or barely proceed to an actual visual inspection of the visible security features printed, applied or otherwise provided onto the security documents.
In other words, the characteristic visual features intrinsic to the processes used for producing the security documents (especially intaglio patterns, line offset patterns, letterpress patterns and/or optically-diffractive structures) have barely been exploited in the context of machine-based authentication.
An exception is the so-called ISARD technology, which was invented and developed by TNO Institute of Applied Physics in the late sixties on behalf of the National Bank of the Netherlands. ISARD stands for Intaglio Scanning And Recognition Device and is based on a measurement of the characteristic relief profile of intaglio-printed features. A discussion of this authentication principle may for instance be found in the following papers:                [Ren96] Rudolf L. van Renesse, “Optical Inspection techniques for Security Instrumentation”, IS&T/SPIE's Symposium on Electronic Imaging, Optical Security and Counterfeit Deterrence Techniques I, San José, Calif., USA (Jan. 28-Feb. 2, 1996), Proceedings of SPIE vol. 2659, pp. 159-167;        [Hei00] Hans A. M. de Heij, De Nederlandsche Bank NV, Amsterdam, the Netherlands, “The design methodology of Dutch banknotes”, IS&T/SPIE's 12th International Symposium on Electronic Imaging, Optical Security and Counterfeit Deterrence Techniques III, San José, Calif., USA (Jan. 27-28, 2000), Proceedings of SPIE vol. 3973, pp. 2-22; and        [Hei06] Hans A. M. de Heij, De Nederlandsche Bank NV, Amsterdam, the Netherlands, “Public feedback for better banknote design”, IS&T/SPIE's International Symposium on Electronic Imaging, Optical Security and Counterfeit Deterrence Techniques VI, San José, Calif., USA (Jan. 17-19, 2006), Proceedings of SPIE vol. 6075, 607501, pp. 1-40.        
The ISARD authentication principle and a device for carrying out this principle are also disclosed in patent publications GB 1 379 764 (corresponding to NL 7017662), NL 7410463, NL 9401796 and NL 9401933.
A problem with the ISARD approach is that it is highly dependent on the degree of wear and use of the documents and the presence of wrinkles in the substrate of the banknotes, which elements directly affect the actual relief profile on the intaglio imprints and its detection by ISARD. ISARD technology was for instance applied as a pattern of parallel intaglio-printed lines on the Dutch 50 guilder “Sunflower” note (issued in 1982), as well as on the current issue of Euro banknotes (see [Hei06]). In practice, the ISARD was and is mainly exploited by the public at large to perform a nail scratching test (i.e. by scratching a nail over the pattern of parallel intaglio lines).
Further solutions to fight counterfeiting and possibly enable machine-based authentication may consist in integrating specific authentication coding in the security document itself, for instance by using specific taggant materials, such as rare-earth components incorporated in the inks or embedded in the paper, or by hiding the authentication coding in the printed patterns themselves using so-called digital watermarking techniques. The integration of specific authentication coding in the security document however implies a specific processing of the document during the design and/or production phase, and a corresponding specifically-designed authentication technique. This accordingly increases the burden on the designer and/or printer to adapt the design process and/or production process of the security documents, and also means that specific detection technology has to be used for the purpose of the authentication process.
A solution based on the integration of specific coding in a printed pattern is for instance disclosed in European patent application EP 1 864 825 A1 (which corresponds to the entry into the European phase of International application No. WO 2006/106677 A1) discloses a printed product and method for extracting information from the printed product wherein information is embedded (or coded) in a printed design, especially a guilloche pattern, in such a way that this information can be detected by subjecting a sample image of the pattern to a Fourier transform. Coding of the information is achieved by spatially modulating the spacing between parallel/concentric curvilinear image elements. Such spatial modulation leads to the production of spectral peaks in the Fourier-transformed spectral image of a sample image of the pattern, which spectral peaks are indicative of the information embedded in the printed design and can thus be decoded. More precisely, according to European patent application EP 1 864 825 A1, the encoded information is extracted by looking at the spectral peak intensities.
A disadvantage of this approach resides in the fact that a specific coding must be embedded in a particular way in the printed patterns to permit decoding. This accordingly imposes substantial restrictions upon the designer who must follow specific design rules to design the printed patterns. In practice, the teaching of European patent application EP 1 864 825 A1 is basically limited to the embedding of information in guilloche patterns as this can readily be seen from looking at the Figures of EP 1 864 825 A1.
The approach disclosed in European patent application EP 1 864 825 A1 is for instance applied with a view to encode information on a personal certificate (such as an identity card, driver licence, or the like), which information relates to the owner/bearer of the personal certificate. The owner-dependent information is encoded into a guilloche pattern printed onto the personal certificate. This accordingly makes it more difficult for counterfeiters to produce similar personal certificates as the information embedded in the guilloche pattern is user-dependent. However, any copy of the personal certificate produced at a similar resolution as the original will exhibit exactly the same information as the original. This approach is thus mainly suitable for the purpose of authenticating security documents intended to bear user-dependent information (which is not the case of banknotes for instance).
U.S. Pat. No. 5,884,296 discloses a device for discriminating an attribute of an image in a block area contained in a document image, which device involves performing a Fourier transformation based on image data in the block area and determining a spatial frequency spectrum relating to the image in the block area. A neural network is exploited to output a discrimination result as to whether or not the attribute of the image in the block area is a halftone dot image based on the spatial frequency spectrum outputted from the Fourier transformation. This device is in particular intended to be used in digital copying machines for the purpose of improving image quality. The device of U.S. Pat. No. 5,884,296 is more particularly intended to be used in the context of the copying of documents containing a mixture of text images, photographic images and/or dot images, which attributes needs be processed separately to yield good image quality in the copied documents. U.S. Pat. No. 5,884,296 does not in any way deal with the issue of authenticating security documents, but rather relates to a solution aimed at improving the discrimination between different attributes of an image.
European patent application No. EP 1 484 719 A2 discloses a method for developing a template of a reference document, such as a banknote, and using that template to validate other test documents, especially for validating currency in an automated teller machine. The method involves using images of a plurality of reference documents, such as genuine banknotes, and segmenting each image in a like manner into a plurality of segments. Each segment is classified using a one-class classifier to determine a reference classification parameter. These parameters are used to define a threshold reference classification parameter. Validation of test documents is thus performed by comparing images of the test documents with the generated template rather than by looking at the intrinsic features of the test documents.
There is therefore a need for a simpler and more efficient approach, especially one that does not as such make use of new design and/or production processes, but rather tries to exploit the intrinsic features of security features that are already typically present on most genuine banknotes, especially the characteristic and intrinsic features of intaglio-printed patterns.